Electrostimulation devices of the above-specified kind are already known in particular in the form of implantable cardiac pacemakers. Such pacemakers in the implanted and operable condition are usually connected by way of an electrode line of the above-specified kind to an electrode disposed in the heart, and adapted to deliver electrical stimulation pulses to the heart by way of the electrode. Those stimulation pulses serve to excite the heart tissue or myocardium and depending on the respective kind of pacemaker are delivered in particular when the heart does not contract naturally at the right time. A contraction is then caused by an electrical pulse which is delivered to the myocardium. If that electrical pulse is of sufficient magnitude, it has the effect that the heart muscle tissue is locally depolarised and correspondingly contracts. Depolarisation and contraction of the heart muscle tissue is intended to extend over all of the stimulated heart muscle and thus lead to the desired contraction of the corresponding chamber of the heart.
The corresponding electrical stimulation pulse must be of a stimulation intensity which is above a respective stimulus threshold of the heart muscle tissue. In that respect the stimulus threshold is a measurement in respect of the minimum stimulation intensity which is sufficient to cause depolarisation of the myocardium and thus a contraction of a respective chamber of the heart. The stimulus threshold depends on various factors and in addition is also variable under some circumstances with the passage of time. With the requirement of delivering a stimulation pulse of adequate stimulation intensity, there is also the need to minimise the energy to be applied for a stimulation pulse. That energy is usually taken from a battery of the cardiac pacemaker which becomes exhausted as time passes. When that battery is exhausted an implanted cardiac pacemaker has to be replaced by a new one in an operative procedure. It is therefore desirable for the cardiac pacemaker to have the longest possible operating time and thus for the battery to have the longest possible service life. In addition the energy for a stimulation pulse should also therefore be as low as possible but sufficient to stimulate only the myocardium but not surrounding muscle tissue.
There is therefore on the one hand the requirement that the intensity of stimulation of a stimulation pulse must suffice to trigger a contraction of the heart muscle tissue. A higher level of stimulation intensity, with other influencing parameters unaltered, entails a higher degree of energy consumption. The stimulation intensity depends on the one hand on the duration of a stimulation pulse and on the other hand on the strength of a stimulation pulse. The strength of a stimulation pulse in turn depends on the electrical voltage with which a stimulation pulse is delivered to the heart muscle tissue. Therefore, a higher level of stimulation intensity usually also results in a higher degree of energy consumption. In order to achieve reliable stimulation of the heart muscle tissue, stimulation pulses are regularly delivered, which from the point of view of energy cost rather more energy than would be necessary as a minimum.
On the other hand, there is a need for the energy consumption per stimulation pulse to be kept as low as possible as that energy is taken from a battery of the cardiac pacemaker, which in that way becomes exhausted.
There is therefore the need to satisfy the requirements for a level of stimulation intensity which is as low as possible and at the same time regular successful stimulation, by optimisation of the stimulation intensity. For that purpose, it is known from the state of the art, for example from U.S. Pat. Nos. 5,350,410; 5,411,533; 5,431,639; and 5,674,254, after delivery of a stimulation pulse, to detect the stimulation outcome (capture) (this being referred to as capture recognition), in order to trigger a back-up stimulation pulse if possible in the event of a defective stimulation outcome or capture.
It is also known for a stimulation outcome to be detected by detecting the impedance of the heart muscle tissue. In this connection attention is to be directed to U.S. Pat. Nos. 5,713,933; 5,735,883; and 5,766,230, and European patent application No. 0 399 063.
In spite of the many known proposals for stimulation outcome monitoring and for automatically adapting the stimulation pulse intensity, there is still the need for a device which is reliable in that respect.
The object of the invention is to afford a suitable device at least as an alternative to the known state of the art.